President Dilma Rousseff of Brazil said on Tuesday that her vice president was orchestrating a conspiracy to topple her, as efforts to impeach her gained momentum in the National Congress.

Aided by her mentor and predecessor, Luiz Inácio Lula da Silva, Ms. Rousseff scrambled to secure enough support from a dwindling array of allies to block impeachment in a lower-house vote set for Sunday that analysts predicted she would lose.

A congressional committee voted on Monday by a larger-than-expected margin to recommend that she be impeached for breaking budget laws to support her re-election in 2014, a charge Ms. Rousseff says was trumped up to remove her from office.